Control method for a vehicle having an engine and an accessory device

ABSTRACT

A method for controlling cycling of an air conditioning compressor coupled to an internal combustion engine interrupts normal cycling based on operation conditions. In addition, normal engaged and disengaged cycling durations are adaptively estimated in real-time. The method of the present invention achieves improved fuel economy and improved drive feel. As an example, improved fuel economy is achieved by engaging the compressor during braking or when the engine is being driven by the vehicle. As another example, improved drive feel is achieved by engaging the compressor during transient conditions when drive feel is unaffected.

This is a Divisional of application Ser. No. 09/482,477, filed on Jan.13, 2000, now ABN.

FIELD OF THE INVENTION

The field of the invention relates generally to air conditioning systemcontrol coordinated with engine control.

BACKGROUND OF THE INVENTION

Vehicles are typically equipped with an air conditioning system toprovide cabin cooling and to dry air for dehumidifying functions. Airconditioning systems typically include a compressor driven by avehicle's internal combustion engine. The compressor can be eitherengaged, fully or partially, or disengaged to the engine via anelectronically controlled clutch.

During air conditioning system operation under certain operatingconditions, the compressor cycles between an engaged and disengagedstate. Cycling is typically controlled based on refrigerant pressure inthe air conditioning system. When the engine and clutch are coupled,pressure decreases and significantly cooled cabin air is circulatedthrough the vehicle. Such operation continues until pressure reaches aminimum value where the clutch is controlled to disengage the engine andcompressor. If air circulation is continued, pressure increases until itreaches a maximum value. At this maximum value, the compressor is thenre-engaged via the clutch and cycling repeats.

It is also known to disengage the engine and compressor during vehiclelaunch conditions, thereby allowing more engine output. In this way,degraded vehicle launch performance is avoided, even when airconditioning is operational. Vehicle launch is determined based onvehicle speed, throttle position, and various other factors.

The inventors herein have recognized disadvantages with the aboveapproaches. First, driver comfort is degraded during clutch engagementsduring some driving conditions. In other words, during some drivingconditions, clutch engagements are felt strongly by vehicle operatorsand comfort is therefore degraded. Second, optimum fuel economy is notobtained since compressor cycling engagement is not coordinated tovehicle and engine operating conditions. In other words, during someconditions, extra fuel is added to the engine to provide airconditioning while maintaining engine output at a desired level. Duringother conditions, no extra fuel is needed to provide air conditioning.

SUMMARY OF THE INVENTION

An object of the present invention is to provide methods for controllingengagements of an air conditioning compressor coupled to an internalcombustion engine capable of improving fuel economy and/or improvingdrive feel.

The above object is achieved and disadvantages of prior approachesovercome by a control method for use with an internal combustion engineand an accessory device, the engine and device coupled to a vehicle, themethod comprising: determining when the device is cycling between anengaged state where the engine is coupled to the device and a disengagedstate where the engine is de-coupled from the device; and engaging thedevice based at least on an operating condition when the device isdisengaged.

By engaging the device in response to an operating condition when thedevice is cycling between an engaged state and a disengaged, it ispossible to coordinate cycling of the device with current drivingconditions. In other words, rather than asynchronous operation betweenvarious control systems, the present invention provides a method tocouple device cycling control to other conditions.

An advantage of the above aspect of the invention is that improved fueleconomy is achieved.

Another advantage of the above aspect of the invention is that improveddrive feel is achieved. In another aspect of the invention, the aboveobject is achieved and disadvantages of prior approaches overcome by acontrol method for use with an internal combustion engine and an airconditioning compressor, the engine and compressor coupled to a vehicle,the method comprising: indicating a transient vehicle driving conditionwhile the vehicle moving; estimating a duration of a cycle in which thedevice is engaged and disengaged due to an air conditioning systemparameter; and engaging the compressor in response to said indicationwhen said duration is greater than a predetermined duration.

By coordinating engagement with a transient vehicle driving conditionwhile the vehicle moving, it is possible to engage the compressorunbeknownst to the vehicle driver. Further, by performing engagementwhen a percentage disengaged duration is greater than a predeterminedduration, it is possible to prevent excessive compressor cycling.

An advantage of the above aspect of the invention is that improved drivefeel and improved customer satisfaction is achieved.

In yet another aspect of the invention, the above object is achieved anddisadvantages of prior approaches overcome by an article of manufacturecomprising a computer storage medium having a computer program encodedtherein for use with an internal combustion engine and an airconditioning compressor, the engine and device coupled to a vehiclehaving brakes. The computer storage medium comprises code fordetermining when the compressor is cycling between an engaged statewhere the engine is coupled to the compressor and a disengaged statewhere the engine is de-coupled from the compressor, code for indicatingwhen the brakes are actuated, code for estimating a percentagedisengaged duration of a cycle in which the compressor is engaged anddisengaged due to an air conditioning system parameter, and codeengaging the compressor based at least on said indication when saidpercentage disengaged duration is greater than a predetermined value.

By engaging the compressor in response to brake actuation when apercentage disengaged duration is greater than a predetermined value, itis sometimes possible to operate the compressor without added fuel tothe engine since kinetic energy from the vehicle can be used to powerthe compressor. In other words, this added coordination betweencompressor cycling control and vehicle braking conditions provides moreopportunities to operate the compressor without excess fuel to theengine.

An advantage of the above aspect of the invention is that improved fueleconomy is achieved.

Other objects, features and advantages of the present invention will bereadily appreciated by the reader of this specification.

BRIEF DESCRIPTION OF THE DRAWINGS

The object and advantages of the invention claimed herein will be morereadily understood by reading an example of an embodiment in which theinvention is used to advantage with reference to the following drawingswherein:

FIG. 1A is a block diagram of a vehicle illustrating various componentsrelated to the present invention;

FIG. 1B is a block diagram of an engine in which the invention is usedto advantage;

FIGS. 2-17 are block diagrams of embodiments in which the invention isused to advantage; and

FIG. 18 is a graph showing an example of operation according to severalaspects of the invention.

DESCRIPTION OF THE INVENTION

Referring to FIG. 1A, internal combustion engine 10, further describedherein with particular reference to FIG. 1B, is shown coupled to torqueconverter 11 via crankshaft 13. Torque converter 11 is also coupled totransmission 15 via turbine shaft 17. Torque converter 11 has a bypassclutch (not shown) which can be engaged, disengaged, or partiallyengaged. When the clutch is either disengaged or partially engaged,torque converter 11 is said to be in an unlocked state. Turbine shaft 17is also known as transmission input shaft. Transmission 15 comprises anelectronically controlled transmission with a plurality of selectablediscrete gear ratios. Transmission 15 also comprises various other gearssuch as, for example, a final drive ratio (not shown). Transmission 15is also coupled to tire 19 via axle 21. Tire 19 interfaces the vehicle(not shown) to the road 23.

Internal combustion engine 10 comprising a plurality of cylinders, onecylinder of which is shown in FIG. 1B, is controlled by electronicengine controller 12. Engine 10 includes combustion chamber 30 andcylinder walls 32 with piston 36 positioned therein and connected tocrankshaft 13. Combustion chamber 30 communicates with intake manifold44 and exhaust manifold 48 via respective intake valve 52 and exhaustvalve 54. Exhaust gas oxygen sensor 16 is coupled to exhaust manifold 48of engine 10 upstream of catalytic converter 20. In a preferredembodiment, sensor 16 is a HEGO sensor as is known to those skilled inthe art.

Intake manifold 44 communicates with throttle body 64 via throttle plate66. Throttle plate 66 is controlled by electric motor 67, which receivesa signal from ETC driver 69. ETC driver 69 receives control signal (DC)from controller 12. Intake manifold 44 is also shown having fuelinjector 68 coupled thereto for delivering fuel in proportion to thepulse width of signal (fpw) from controller 12. Fuel is delivered tofuel injector 68 by a conventional fuel system (not shown) including afuel tank, fuel pump, and fuel rail (not shown).

Engine 10 further includes conventional distributorless ignition system88 to provide ignition spark to combustion chamber 30 via spark plug 92in response to controller 12. In the embodiment described herein,controller 12 is a conventional microcomputer including: microprocessorunit 102, input/output ports 104, electronic memory chip 106, which isan electronically programmable memory in this particular example, randomaccess memory 108, and a conventional data bus.

Controller 12 receives various signals from sensors coupled to engine10, in addition to those signals previously discussed, including:measurements of inducted mass air flow (MAF) from mass air flow sensor110 coupled to throttle body 64; engine coolant temperature (ECT) fromtemperature sensor 112 coupled to cooling jacket 114; a measurement ofthrottle position (TP) from throttle position sensor 117 coupled tothrottle plate 66; a measurement of transmission shaft torque, or engineshaft torque from torque sensor 121, a measurement of turbine speed (Wt)from turbine speed sensor 119, where turbine speed measures the speed ofshaft 17, and a profile ignition pickup signal (PIP) from Hall effectsensor 118 coupled to crankshaft 13 indicating an engine speed (We).Alternatively, turbine speed may be determined from vehicle speed andgear ratio.

Continuing with FIG. 1, accelerator pedal 130 is shown communicatingwith the driver's foot 132. Accelerator pedal position (PP) is measuredby pedal position sensor 134 and sent to controller 12.

In an alternative embodiment, where an electronically controlledthrottle is not used, an air bypass valve (not shown) can be installedto allow a controlled amount of air to bypass throttle plate 62. In thisalternative embodiment, the air bypass valve (not shown) receives acontrol signal (not shown) from controller 12.

In a preferred embodiment, controller 12 controls engine according to atorque based control system. In such a system, a desired wheel torque,or engine torque, is determined based on pedal position (PP). Then,position of throttle 66 is controlled so that actual wheel torque, orengine torque, approaches the desired engine torque. The system can beconfigured based on engine brake torque, which is the available torqueat the engine output, taking into account torque losses.

Referring now to FIG. 2, an air conditioning (A/C) system is shown.Arrows 201 indicate direction of refrigerant, or working fluid, flow.Arrows 200 indicate direction of air flow that is circulated through theengine compartment (not shown). Arrows 206 indicate direction of airflow that is circulated through the cabin (not shown). Solid shading 202indicates working fluid is a high pressure gas, left handedcross-hatching 203 indicates working fluid is a high pressure liquid,right handed cross-hatching 204 indicates working fluid is a lowpressure liquid, and no shading 205 indicates working fluid is a lowpressure gas. Working fluid is circulated through the A/C system vialine 207. Compressor 220, which can be coupled to engine 10 via a clutch219, is located between high pressure gas 202 and low pressure gas 205.Upstream of compressor 220 is low pressure service port 222 and A/Ccycling pressure switch 223. Upstream of cycling switch 223 is suctionaccumulator/drier 224. Further upstream of suction accumulator/drier 224is A/C evaporator core 226, which is coupled to blower motor 225.Continuing upstream of A/C evaporator core 226 is A/C evaporator orifice227 and A/C condenser core 228, which is coupled to radiator fan 233.Upstream of A/C condenser core 228 is high pressure service port 229,compressor relief valve 230, and A/C pressure cut-off switch 231.

A description of an A/C thermodynamic process is now presented. Startingat compressor 220, low pressure gas 205 is compressed to high pressuregas 202, rising in temperature due to compression. Compressor reliefvalve 230 prevents high pressure gas 202 from reaching a maximumallowable high pressure gas pressure. A/C pressure cut-off switch 231disengages compressor 200 from engine 10 via clutch 219.

High pressure gas 202 sheds heat to the atmosphere at A/C condenser core228, changing phase to high pressure liquid 203 as it cools. At A/Cevaporator orifice 227, high pressure liquid 204 expands to low pressureliquid 204. At A/C evaporator core 226 low pressure liquid 204 passesthrough a jet (not shown) and evaporates into low pressure gas 205. Thisaction cools the working fluid, A/C evaporator core 226, and cabinairflow 206.

Low pressure liquid 204 continues to suction accumulator/drier 224 andA/C cycling pressure switch 223. A/C cycling pressure switch 223 signalsto engage compressor 220 to engine 10 via clutch 219 when measuredpressure is above a predetermined maximum pressure. A/C cycling pressureswitch 223 also signals to disengage compressor 220 from engine 10 viaclutch 219 when measured pressure is below a predetermined minimumpressure. These setpoint pressures are typically 45 psi and 24.5 psi,respectively. They are designed to keep A/C evaporator core 226 justabove freezing. When compressor 220 cycles between engaged anddisengaged due solely to A/C cycling pressure switch 223, it is referredto herein as normal, or uninterrupted, cycling. Stated another way, thisnormal/uninterrupted cycling is when the compressor cycles to controlcabin temperature, or cooling air temperature, based on air conditioningparameters such as pressure or temperature. However, according to thepresent invention, engagement of compressor 220 is controlled due tovarious factors as described later herein.

Referring to FIG. 3, a routine is shown for learning the on and offduration of A/C compressor 201. First, in step 300, a determination ismade as to whether the A/C system is presently cycling. In other words,engagement due to engine operating conditions is not enabled unlesscompressor 201 has cycled a predetermined number of times. When theanswer to step 300 is YES, the routine continues to step 302. In step302, a determination is made as to whether A/C compressor 201 has justdisengaged. In other words, a determination is made as to whether A/Ccompressor 201 has just been disconnected from engine 10. When theanswer to step 302 is YES, a determination is made in step 304 as towhether the A/C compressor was engaged due to normal cycling. In otherwords, a determination is made as to why the compressor was previouslyengaged. If it was engaged due to normal cycling, which means pressuremeasured by sensor 223 was greater than a predetermined value, then theroutine continues to step 306. Stated another way, if an uninterruptedcycle was completed, it is possible to learn normal on and offdurations. In step 306, the routine calculates temporary values A′ andB′ from the previous cycle. Value A′ represents the duration that A/Ccompressor 201 was engaged and B′ represents the duration A/C compressor201 was disengaged. In other words, A′ and B′ respectively represent onand off durations for normal cycling under present operating conditions.Next, in step 308, the learned values A and B are updated based on thecalculated temporary values A′ and B′ using filter coefficients γ1 andγ2. In other words, the learned on and off durations are filtered toremove measurement noise. When the answer to step 304 is NO, the routinecontinues to step 310, where values A and B are not updated. Inaddition, if compressor 201 was disengaged due to vehicle launchconditions, the routine continues to step 310. In this way, it ispossible to learn the on and off durations of uninterrupted (or normal)A/C compressor cycling with the present conditions. In other words, theon and off durations are adaptively learned for normal (uninterrupted)A/C operation.

In an alternative embodiment, values A and B are learned as a functionof air conditioning operating conditions such as, for example, blowerspeed, desired cabin temperature, desired cooling level, ambienttemperature, cabin humidity, and/or ambient humidity. By includingvariation in these air conditioning operating conditions, values A and Bfor current operating conditions can be used to include an open loopestimate to account for quickly changing driver requests or quicklychanging ambient conditions.

Referring now to FIG. 4A, a routine is described for determining whethernormal A/C cycling can be interrupted to engage A/C compressor 201.First, in step 410, the time A/C compressor 201 has been off, or thetime since A/C compressor 201 was last disengaged, is measured (cur_b).Next, in step 412, the percent of an uninterrupted cycle in which theA/C compressor has been off, is calculated. In other words, the routinecalculates the percent of an uninterrupted cycle that A/C compressor 201has been off (pb) at the present calculation point. This value iscalculated based on the time measured in step 410 (cur_b) and thelearned off-time (B). Next, in step 414, a determination is made as towhether the value pb is greater than a limit value (pb_limit). Statedanother way, engagement due to operating conditions is prevented untilcompressor 201 has been disengaged for a predetermined duration. In thisparticular example, the duration is a relative percentage of thepresently estimated off duration (B). This prevents excessive cycling.For example, if compressor 201 is engaged right after it was disengaged,it will again be disengaged since measured pressure will quickly reachthe maximum limit value. When the answer to step 414 is NO, anengagement flag (engage_flg) is set equal to zero in step 416.Otherwise, in step 418, an engagement flag is set equal to 1. In otherwords, in step 418, the routine enables A/C engagement due to variousconditions described later herein.

In an alternative embodiment of the present invention, step 414 can bemodified to determine whether time measured in step 410 (cur_b) isgreater than a predetermined limit time (cur_b_limit). Those skilled inthe art will recognize various other methods to prevent excessivecycling such as determining if compressor 201 has been off for apredetermined number of engine rotations.

Referring now to FIGS. 4B-4D, several graphs show an example ofoperation according to the present invention. FIG. 4B shows whether A/Ccompressor 201 is engaged or disengaged as well as on and off durationsA′ and B′, respectively. FIG. 4C shows the corresponding percent of anuninterrupted cycle that A/C compressor 201 has been off (pb). Also,limit value (pb_limit) is shown by a dash dot line. FIG. 4D showscorresponding engagement flag (engage_flg). According to the presentinvention as described with particular reference to FIG. 4A, when pb isgreater than pb_limit, engage_flg is set equal to one. Otherwise,engage_flg is set equal to zero.

The A/C compressor cycling of the present invention is controlled byvarious parameters. Uninterrupted A/C compressor cycling, as definedherein, represents when the A/C compressor is cycled on and off based onpressure measured by A/C cycling pressure switch 203. This uninterruptedcycling is also referred to herein as normal cycling. In this normalcycling, the A/C compressor engages and disengages so that the driver isprovided with requested cooling. Further, in this normal cycling, theA/C compressor is engaged when the A/C cycling pressure switch 203measures a pressure greater than a first predetermined value. The A/Ccompressor stays on until the A/C cycling pressure switch 203 measures apressure less than a second value. At this point, the A/C compressor isdisengaged. The A/C compressor remains disengaged until, once again, A/Ccycling pressure switch 203 measures a pressure greater than the firstvalue. In this way, the A/C cycles normally on and off based onenvironmental conditions and driver requests.

According to the present invention, engagement of the A/C compressor isalso performed under various other conditions. These conditions can betransient vehicle operating conditions; conditions where the A/Ccompressor can be driven with minimal fuel economy impact; andconditions where the potential for minimum drive impact during theengagement is possible. The following figures describe such operation.

Referring now to FIG. 5, a routine is described for determining whetherto engage the A/C compressor. First, in step 500, a determination ismade as to whether the A/C system is presently cycling. In other words,engagement due to engine operating conditions is not enabled unlesscompressor 201 has cycled a predetermined number of times. When theanswer to step 500 is YES, in step 510, a determination is made as towhether the engagement flag (engage_flg) is set equal to 1. When theanswer to step 510 is YES, a determination is made in step 512 as towhether enabling conditions have been detected based on engine orvehicle conditions (see FIG. 6). When the answer to step 512 is YES, theA/C compressor is engaged in step 514 and interrupt flag (int_flag) isset equal to 1.

When the answer to step 510 is NO, a determination is made in step 516as to whether A/C cycling pressure switch 203 indicates that A/Cengagement is necessary. When the answer to step 516 is YES, in step 518the A/C compressor is engaged, interrupt flag (int_flag) is set equal tozero, and normal cycling will follow.

Referring now to FIG. 6, a routine is described for determining whetherenabling conditions have been detected. First, in step 610, it isdetermined whether vehicle speed is greater than vehicle speed threshold(pvs). When the answer to step 610 is YES, a determination is made as towhether transient conditions have been detected in step 612. Thedetection of transient conditions is described later herein. When theanswer to step 612 is NO, a determination is made in step 614 as towhether there is a potential for more efficient A/C operation.Determining whether more efficient A/C operation is possible isdescribed later herein. When the answer to step 614 is NO, adetermination is made in step 616 as to whether there is a potential forminimum drive impact during engagement. When the answer to either step612, 614 or 616 is YES, the routine indicates in step 618 that enablingconditions have been detected.

Other conditions can also be used in determining whether to enableengagement according to the present invention. For example, during highambient temperatures, cycling is minimal. Stated another way, ifcompressor 220 if cycled off only for less than a minimal off time,enabling conditions would not be detected.

Referring now to FIG. 7, a routine is described for determining whetherpotential for more efficient A/C operation has been detected. First, instep 710, a determination is made as to whether torque converter 11 isunlocked. When the answer to step 710 is YES, a speed ratio (sr) iscalculated across torque converter 11 based on engine speed (We) andturbine speed (Wt). Next, in step 714, a determination is made as towhether the calculated speed ratio is less than 1. When the answer tostep 714 is YES, the routine indicates in step 716 that there is apotential for more efficient A/C operation. In other words, when torqueconverter speed ratio is less than 1, the engine is absorbing torque, orengine brake torque is less than zero, and therefore it is possible toengage the A/C compressor and use the force transmitted from the roadthrough the engine powertrain to power the A/C compressor. In this way,less fuel is used since the A/C compressor is not being powered byengine combustion. Stated another way, excess fuel does not need to beadded to the engine to power the A/C compressor when the engine isproducing negative engine brake torque, i.e., when the engine is beingdriven.

Referring now to FIG. 8, a routine is described for detecting transientconditions. First, in step 810, a determination is made as to whether anantilock braking system is activated. When the answer to step 810 isYES, the routine indicates in step 812 that transient conditions havebeen detected. In other words, when antilock braking systems areactivated the hydraulic pulsing that applies the hydraulic brakeactuator interrupts normal drive feel experienced by the vehicleoperator. Therefore, if the A/C compressor is engaged while the antilockbraking system is activated, the driver will not notice the A/Cengagement. In this way, the A/C compressor can be engaged less oftenduring normal drive situations where the driver may feel the A/Ccompressor engagement. Thus, drive feel is improved.

Referring now to FIG. 9, a routine is described for detecting transientconditions. First, in step 910, a determination is made as to whethertraction control is engaged. When the answer to step 910 is YES, theroutine indicates in step 912 that transient conditions have beendetected. In other words, when traction control systems are activated,application of brakes and/or reduction in engine torque interrupts thenormal drive feeling experienced by the vehicle operator. Therefore, ifthe A/C compressor is engaged while the traction control system isactivated, the driver will not notice the A/C engagement. In this way,the A/C compressor can be engaged less often during normal drivesituations where the driver may feel the A/C compressor engagement.Thus, drive feel is improved.

Referring now to FIG. 10, a routine is described for detecting transientconditions. First, in step 1010, a determination is made as to whethercruise control was commanded to be engaged or disengaged. When theanswer to step 1010 is YES, the routine indicates in step 1012 thattransient conditions have been detected. In other words, when cruisecontrol is activated or deactivated, the change in control from thedriver to the automatic control system or from the automatic controlsystem to the driver can interrupt normal drive feel experienced by thevehicle operator. Therefore, if the A/C compressor is engagedconcurrently with engagement or disengagement of the cruise controlsystem, the driver will not notice the A/C engagement. In this way, theA/C compressor can be engaged less often during normal drive situationswhere the driver may feel the A/C compressor engagement. Thus, drivefeel is improved.

Referring now to FIG. 11, a routine is described for detecting potentialfor minimum drive impact during engagement. First, in step 1110, adetermination is made as to whether torque converter 11 is unlocked.When the answer to step 1110 is YES, the routine continues to step 1112,where a determination is made as to whether the percent off-time (pb) isgreater than limit value (pb_limit_uc). When the answer to step 1112 isYES, the routine indicates in step 1114 that the potential for minimumdrive impact during engagement has been detected. In other words, it isless likely that a vehicle operator will feel A/C compressor engagementswhen torque converter 11 is unlocked since additional damping isprovided by an unlocked torque converter. Thus, if the A/C compressorhas been disengaged for greater than limit value (pb_limit_uc), improveddrive feel can be achieved by taking advantage of the current situationand engaging the A/C compressor, rather than waiting until the A/Ccycling pressure switch 203 indicates that the A/C compressor should beengaged due to measured pressure.

Referring now to FIG. 12, a routine is described for detecting transientconditions. First, in step 1210, a determination is made as to whether atransmission shift has been commanded or detected. When the answer tostep 1210 is YES, the routine indicates in step 1212 that transientconditions have been detected. In other words, during a transmissionshift vehicle acceleration or deceleration can occur, interruptingnormal drive feel experienced by the vehicle operator. Therefore, if theA/C compressor is engaged during a transmission shift, the driver willnot notice the A/C engagement since the driver expects vehicle feel tochange. In this way, the A/C compressor can be engaged less often duringnormal drive situations where the driver may feel the A/C compressorengagement. Thus, drive feel is improved.

Referring now to FIG. 13, a routine is described for detecting thepotential for minimum drive impact during engagement. First, in step1310, a determination is made as to whether desired engine brake torqueis less than zero. For example, to control vehicle speed to a desiredvehicle speed during cruise control on a steep downgrade, it may benecessary to provide engine braking. Alternatively, if vehicleacceleration is controlled to a desired acceleration, negative enginebrake torque may be requested. When the answer to step 1310 is YES, theroutine indicates in step 1312 that potential for more efficient A/Coperation is detected. In other words, when engine 10 is absorbingtorque, it is possible to engage the A/C compressor and use forcetransmitted from the road through the engine powertrain to power the A/Ccompressor. In this way, less fuel is used since the A/C compressor isnot being powered by engine combustion. Stated another way, excess fueldoes not need to be added to the engine to power the A/C compressor whenthe engine is producing negative engine brake torque.

In an alternative embodiment, potential for more efficient A/C operationcan be determined directly from a desired vehicle acceleration. Forexample, if desired vehicle acceleration (which can be determined basedon pedal position (PP)) is negative, or is less than a predeterminedacceleration, potential for more efficient A/C operation can beindicated.

Referring now to FIG. 14, a routine is described for detecting thepotential for minimum drive impact during engagement. First, in step1410, a determination is made as to whether deceleration fuel shut-off(DFSO), or partial cylinder deactivation, is active (or requested). Forexample, to control vehicle speed to a desired vehicle speed duringcruise control on a steep downgrade, it may be necessary to providesignificant engine braking to a point where combustion in some enginecylinders is terminated. When the answer to step 1410 is YES, theroutine indicates in step 1412 that potential for more efficient A/Coperation is detected. In other words, when engine 10 is absorbingtorque, it is possible to engage the A/C compressor and use forcetransmitted from the road through the engine powertrain to power the A/Ccompressor. In this way, less fuel is used since the A/C compressor isnot being powered by engine combustion. Stated another way, excess fueldoes not need to be added to the engine to power the A/C compressor whenthe engine is producing negative engine brake torque.

Referring now to FIG. 15, a routine is described for detecting moreefficient A/C operation. First, in step 1510, a determination is made asto whether pedal position (PP) is less than minimum pedal position(MPP). In other words, if the driver has tipped-out, this can be viewedas a request for some deceleration and reduced engine torque. One methodfor reducing engine torque in an efficient manner is to engage the A/Ccompressor. Thus, when the answer to step 1510 is YES, the routineindicates in step 1512 that potential for more efficient A/C operationis detected. In other words, when engine 10 is absorbing torque todecelerate, it is possible to engage the A/C compressor and use thedeceleration force to power the A/C compressor. In this way, less fuelis used since the A/C compressor is not being powered by enginecombustion. Referring now to FIG. 16, a routine is described fordetecting transient conditions. First, in step 1610, a change in pedalposition (Δpp) is calculated. In step 1612, a change in desired enginetorque (ΔT_des) is calculated. Next, in step 1614, a change in throttleposition (ΔTp) is calculated. Next, in step 1616, a change in fuelinjection amount (Δfpw) is calculated. In step 1618, a determination ismade as to whether the absolute value of any of these changes is greaterthan corresponding threshold values. When the answer to step 1618 isYES, the routine continues to step 1620 where the routine indicates thattransient conditions have been detected. In this way, when the vehicleoperator makes a change in power or torque delivered by engine 10, theA/C compressor can be engaged. Also, in this way, it is possible to maskengagement of the A/C compressor, since the driver will be expecting asignificant change in vehicle operation.

Referring now to FIG. 17, a routine is described for detecting thepotential for more efficient A/C operation. First, in step 1710, adetermination is made as to whether vehicle brakes are actuated, forexample by detecting whether the driver pressed a brake pedal. In otherwords, if the driver has applied the brakes this can be viewed as arequest for some deceleration and reduced engine torque. One method forreducing engine torque in an efficient manner is to engage the A/Ccompressor. Thus, when the answer to step 1710 is YES, the routineindicates in step 1712 that potential for more efficient A/C operationis detected. In other words, when engine 10 is absorbing torque todecelerate, it is possible to engage the A/C compressor and use thedeceleration force to power the A/C compressor. In this way, less fuelis used since the A/C compressor is not being powered by enginecombustion. Referring now to FIG. 18, a graph depicting operationaccording to the present invention is shown. The graph shows whether theA/C compressor is engaged or disengaged. The graphs starts at time t0where the A/C compressor is disengaged. The compressor is then engagedafter duration B′ and remains engaged for duration A′. At time t1, theA/C compressor is disengaged. At time t1, the routine is able to measurevalues A′ and B′ and update values A and B, since the normal A/C cyclingoccurred and was not interrupted. At time t2, another uninterrupted A/Ccompressor cycle is completed, and again values A′ and B′ are measuredand values A and B updated. At time t3, the vehicle brakes are applied.At time t3, since the percent of an uninterrupted A/C compressor cycle(pb) is greater than the limit value (pb_limit), the A/C compressor isengaged. At time t4, the A/C compressor is disengaged due to pressuremeasured by A/C cycling pressure switch 203. At time t4, the parametersA and B are not updated since the value of A′ and B′ cannot be measuredsince a normal cycle was not completed. At time t5, a vehicle gear shiftis performed. At time t5, the A/C compressor is not engaged since theoff time (pb) is too small. At time t6, another uninterrupted A/Ccompressor cycle has been completed and the values A′ and B′ can bemeasured so the values A and B can be updated. At time t7, enginebraking is detected and the A/C compressor is engaged since the off time(pb) is greater than the limit value (pb_limit). At time t8, the A/Ccompressor is disengaged based on the pressure measured by A/C cyclingpressure switch 203. At time t9, the brakes are actuated. However, theA/C compressor is not engaged since the off time is too small. At timet10, torque converter 11 is unlocked due to vehicle driving conditions.At time t10, the A/C compressor is engaged since the off time (pb) isgreater than the limit value (pb_limit) and greater than torqueconverter unlocked limit value (pb_limit_uc). At time t11, the A/Ccompressor is disengaged due to pressure measured by A/C cyclingpressure switch 203. Parameter A′ and B′ are not measured at time t11nor are parameters A and B updated. Next, at time t12, anotheruninterrupted A/C compressor cycle has been completed and values A′ andB′ can be measured so that values A and B can be updated.

The graph in FIG. 18 shows an example of operation in which a portion ofthe conditions which can cause the A/C compressor to be engaged aredescribed.

Although several examples of embodiments which practice the inventionhave been described herein, there are numerous other examples whichcould also be described. For example, the invention can also be usedwith direct injection engines wherein fuel is injected directly into theengine cylinder. Also, the invention is applicable with various types ofaccessory devices that can cycle between an engaged state and adisengaged state. In another example, potential for minimum drive impactcan also be indicated when a clutch is depressed (or disengaged) in amanual transmission vehicle. During such a condition, it is may bepossible to engage compressor 220 without affecting drive feel sinceengine 10 is not coupled to the wheels or transmission of the vehicle.The invention is therefore to be defined only in accordance with thefollowing claims.

We claim:
 1. A control method for use with an internal combustion enginecoupled to a drive train and an air conditioning compressor in a vehicledriven by an operator, the compressor coupled to the engine by anelectronically operated clutch device which engages and disengages thecompressor, the method comprising: indicating a transient vehicledriving condition while the vehicle is moving faster than apredetermined vehicle speed, said transient vehicle driving conditionincluding intervention of the drive train interrupting drive feelexperienced by the vehicle operator; estimating a duration of a cycle inwhich the device is engaged and disengaged due to an air conditioningsystem parameter; and engaging the compressor in response to saidindication when said duration is greater than a predetermined duration.2. The method recited in claim 1 wherein said intervention of the drivetrain is selected from the group consisting of engagement of tractioncontrol, engagement or disengagement of cruise control, occurrence of atransmission shift, occurrence of anti-lock braking, and combinationsthereof.
 3. The method recited in claim 1, further comprising engagingthe compressor in response to said air conditioning system parameterless often during other conditions.
 4. A control method for use with aninternal combustion engine and an air conditioning compressor in avehicle, the compressor coupled to the engine by an electronicallyoperated clutch device which engages and disengages the compressor, themethod comprising: determining whether the vehicle is moving faster thana predetermined vehicle speed; indicating a transient vehicle drivingcondition while the vehicle is moving faster than said predeterminedvehicle speed, wherein said transient vehicle driving condition is whena transmission shift occurs; estimating a duration of a cycle in whichthe device is engaged and disengaged due to an air conditioning systemparameter; and engaging the compressor in response to said indicationwhen said duration is greater than a predetermined duration.
 5. Themethod recited in claim 4 wherein said estimating further comprisesestimating a disengaged duration of said cycle in which the device isengaged and disengaged due to said air conditioning system parameter andsaid engaging further comprises engaging the compressor in response tosaid indication when said disengaged duration is greater than saidpredetermined duration.
 6. The method recited in claim 4 wherein saidtransient vehicle driving condition is when a transmission shift iscommanded.
 7. The method recited in claim 4 wherein said transientvehicle driving condition is when a transmission shift is detected. 8.The method recited in claim 4 wherein the engine is coupled to anautomatic transmission.
 9. A control method for use with an internalcombustion engine and an air conditioning compressor, the engine andcompressor coupled to a vehicle, the method comprising: indicatingwhether the vehicle is moving faster than a predetermined vehicle speed;while the vehicle is moving faster than said predetermined vehiclespeed, determining whether a transmission shift has occurred; estimatinga duration of a cycle in which the compressor is engaged and disengageddue to an air conditioning system parameter; and engaging the compressorin response to said transmission shift when said duration is greaterthan a predetermined duration.
 10. The method recited in claim 9 whereinthe engine is coupled to an automatic transmission.
 11. The methodrecited in claim 10 wherein said determining whether said transmissionshift has occurred includes determining whether a transmission shift hasbeen commanded.
 12. The method recited in claim 10 wherein saiddetermining whether said transmission shift has occurred includesdetermining whether a transmission shift is detected.